Multi-layer composites, below designated as “composites” or “composite films,” are used for such things as packaging foods and pharmaceutical products. Frequently the top film or layer of such composites is coated with vinylidene chloride polymer- or copolymer dispersions. For example for the packaging of pharmaceutical products, PVC films or PVC-PE composite films are coated with vinylidene chloride copolymer dispersions and subsequently processed in a thermoforming process into blisters, which are then filled with packing products and are sealed shut with an aluminum foil or a thin polymer cover film. Vinylidene chloride monomers are usually polymerized into copolymers with methacrylates, acrylates, vinyl chloride or acrylonitrile. The purpose of the vinylidene chloride copolymer coating is to form a sealable coating and/or formation of a barrier layer against vapor, oxygen and aromas.
Vinylidene chloride copolymer dispersions form plastic films which feature, when in a fresh condition, and especially if they contain no wax or other lubricant additive, for one a blocking tendency, i.e. a tendency to adhere to a smooth surface, such as the nearest wrap of a film roll, and, for another, very high coefficients of static friction and sliding friction in relation to metals, plastics and organic materials. The coefficients of sliding friction may be reduced after the surface of the PVDC coating has become harder due to more or less quickly on-setting and progressing crystallization. But a tendency to adhere to machine parts, or to poor sliding characteristics when being filled with packing goods, especially with gelatin capsules, remains undiminished. A tendency to unwanted sticking of the PVDC layer to hot machine parts, for example in the heating zone of thermoforming machines, has also been observed.
After the composite film is wrapped the blocking tendency results in the creation of dabbing spots on the soft PVDC surface caused by contact with the adjacent wrap of film. If the PVDC surface is very smooth, as in the case of the screen cylinder or tri-helix roller coating systems usually used for PVDC dispersion coating, if no counteractive measures are taken, the air wrapped over results in large zones with a glossy surface, since the contact with the adjacent wrap is prevented by enclosed air bubbles. Adjacent to these are zones with a matt surface which are in contact with the adjacent wrap and in which, as a result, dabbing spots occur. The dabbing images themselves do not result in a technical fault in the PVDC coating, but such composites are frequently not accepted on aesthetic grounds. However, in many cases, PVDC coated films can also develop an unwanted film deformation, possibly leading to secondary faults due to the wrapped over air bubbles.
Generally this air-enclosure effect is reduced by applying a specific surface structure permitting the air to escape from the sides. Also the dabbing spots, due to the fact that they basically always appear on the wave crests of the surface structure, are distributed evenly across the whole surface of the film, and the film acquires in this manner a more homogenous appearance.
The specific generation of such a surface structure determines the use of additional equipment in a coating machine. The quality of the structure, i.e. the roughness height, appearance, the mean layer thickness of the top layer of the composite, must be controlled by machine settings and, in general, in comparison to a smooth film surface, the generation of a surface structure is accompanied by a loss in film thickness and, thus, by a possible loss in productivity. Due to the surface unevenness, the application of a surface structure onto a PVDC coated film is a disadvantage, or even impossible, if the coated composite is subjected to an additional lamination or coating step on the PVDC side.
Freshly coated and wrapped composite film rolls relax over a period of a few hours or days, in that they collapse somewhat in those sections which are wrapped less tightly, or at places where the film is less thick, and draw tight at places characterized by greater film thickness. One result of the block tendency of the freshly coated PVDCs consists in the fact that, due to the insufficient sliding capacity between the film wraps and to the simultaneous relaxing, tension arises along the film surface which, if the block tendency is too pronounced, can even lead to material destruction caused by the breaking of the composite films into the individual films or layers.
In DE 100 64 800 A1 micronized polyethylene waxes are described which are manufactured by the co-polymerization of ethylene under high-pressure conditions using an aliphatic or aliclycic ketone as molecular weight regulator and subsequent micronization. What is meant by micronization is the breaking up of substances into particle sizes ranging from 1 μm to a maximum of 100 μm. The substances are rendered into the desired morphology by grinding or spraying. There are numerous applications known for micronized polyethylene waxes, for example as carriers for printing inks, coatings, abrasive for toothpastes, and additive for cosmetic preparations such as eyeshadows, lipsticks, or blusher.
DE 43 16 025 A1 relates to micronized polyethylene wax of a mean particle size of 1 to 30 μm, suitable as a solvent to detackify the surface of sticky granulates. The micronized polyethylene wax develops its effect even in small quantities of 0.01 to 2% by weight of the granulate. The micronized polyethylene waxes are made by spraying a relevant polyethylene melt with gases in a two-stage nozzle.
EP 0 403 542 B1 discloses a polymer composition which comprises a vinylidene chloride interpolymer formed from a monomer mixture. The monomer mixture contains 60 to 99% by weight vinylidene chloride and 40 to 1% by weight of at least one ethylene-type unsaturated comonomer which is copolymerizable with it. The vinylidene chloride interpolymer is mixed with a formulation package which contains 0.1 to 95% by weight in relation to the total weight of the formulation package, an alkali metal salt or an alkaline earth metal salt of a weak acid, an ethylene homopolymer and at least one plasticizer and one lubricant. The vinylidene chloride interpolymer can be used to make objects by casting, blowing, extrusion molding, coextrusion, laminating, or calendering of the polymer compound.
EP 0 736 067 B1 describes a process of stabilizing polyolefin mixtures containing PVDC, and stabilized polyolefin mixtures containing PVDC. The plastic mixture comprises polyolefins and 0.05 to 20% by weight PVDC and at least one organic phosphite or phosphonite as well as at least one metal salt of a fatty acid.
DE 198 32 500 A1 relates to a thermoformable composite film which comprises a film containing at least one cycloolefin copolymer and a thermoplastic film laminated onto at least one side of it. The thermoformable film is laminated to the film containing cycloolefin copolymer by a solvent-free, single-component adhesive. The thermoplastic film contains PVDC. The thickness of the film as a whole is 100 to 500 μm, the thickness of the PVC film ranges from 5 to 150 μm, and the thickness of the COC film is 50 to 400 μm. This thermoformable composite film is used to make blister packs. The thermoplastic film can also be selected from polyolefin, polyamide, polyester, polycarbonate, polystyrene, polyvinyl chloride, and polyurethane.